1. Field of the Invention
The present invention relates to a door opening/closing system which comprising a door which is operated by an electric motor.
2. Description of the Related Art
NC lathes and machining centers and other machine tools are often surrounded by external covers which prevent scattering to the surrounding environment of the machining fluid which is used for processing of a workpiece and the swarf which is produced during processing of a workpiece. Normally, the external cover of a machine tool comprises an automatically operating door. The door is closed while the workpiece is processed, but the door is opened while workpieces are changed. Further, a workpiece on a work table is changed by an automatic changing system which is installed adjoining the machine tool. In relation to this, JP-A-2008-207270 proposes a door device for machine tool which comprises a slide door which is driven to open/close by a pneumatic cylinder, and a brake which is driven by another pneumatic cylinder to stop the slide door. More specifically, the door device of JP-A-2008-207270 is designed to start the feed of air to the pneumatic cylinder for the brake, simultaneously with stopping the feed of air to the pneumatic cylinder for the slide door, when the slide door strikes an obstacle. Further, JP-U-H05-74744 proposes an automatically opening/closing door device which comprises a main door which is driven by an air cylinder, and a small sized, light weight subdoor which is driven by an air cylinder with a smaller thrust force than the main door. More specifically, the door device of JP-U-H05-74744 can close the main door only when the subdoor is closed. This ensures that a user is prevented from being caught in the large sized, heavy weight main door.
However, it is difficult to precisely control the operation of a pneumatic door, and therefore, to prevent a pneumatic door from striking an obstacle and being damaged, it is necessary to maintain a low operating speed of the door. For this reason, if the door device of JP-A-2008-207270 is employed, the cycle time for changing a workpiece of a machine tool is liable to become longer. Further, two pneumatic circuits for the slide door and for the brake are necessary in the device of JP-A-2008-207270, and a subdoor is required in addition to a main door in the device of JP-U-H05-74744. Therefore, the device as a whole becomes complicated in structure and its manufacturing cost is increased.
A door operating system comprising a door which can be opened/closed at a high speed, and being capable of quickly stopping the door when it strikes an obstacle, has been sought.